Method of making permanent ferrite magnets

ABSTRACT

A method of making sintered magnetoplumbite ferrite compositions having the formula MFe12O19 wherein M is selected from the group consisting of strontium and barium is provided. The steps comprise forming an aqueous slurry of the ferrite and a liquid phase sintering additive, orienting the ferrite to align the crystals and wet pressing the mixture to form a green compact, sintering the green compact at a temperature of about 1,100*1,300* C. for a sufficient amount of time and magnetizing the sintered ferrite compositions.

United States Patent Arendt et al.

METHOD OF MAKING PERMANENT FERRITE MAGNETS Inventors: Ronald H. Arendt;William J.

Dondalski, both of Schenectady, N.Y.

Assignee: General Electric Company,

Schenectady, NY.

Filed: Apr. 26, 1973 Appl. No.: 354,878

US. Cl. 252/6258; 252/6259; 252/6263; 264/63 Int. Cl. C04B 33/32; C04B35/26; HOlF l/ll Field of Search 252/6258, 62.63, 62.59; 264/24, 62, 63,65

References Cited UNITED STATES PATENTS 4/1966 Blume 264/62 8/l967Cochardt 252/6263 lO/l973 Cochardt et al...... 252/6263 PrimaryExaminer-Donald 1. Arnold Attorney, Agent, or FirmCharles T. Watts;Joseph T. Cohen; Jerome C. Squillaro [57] ABSTRACT A method of makingsintered magnetoplumbite ferrite compositions having the formula MFe Owherein M is selected from the group consisting of strontium and bariumis provided. The steps comprise forming an aqueous slurry of the ferriteand a liquid phase sintering additive, orienting the ferrite to alignthe crystals and wet pressing the mixture to form a green compact,sintering the green compact at a temperature of about l,l0O-l,300 C. fora sufficient amount of time and magnetizing the sintered ferritecompositions.

3 Claims, No Drawings METHOD OF MAKING PERMANENT FERRITE MAGNETS Thisinvention relates to a method of making permanent ferrite magnets and inparticular to materials which have a hexagonal lattice structure and areknown as magnetoplumbite ferrites. The properties of these materials,their crystal structure and the method of preparation have beendisclosed in the prior art.

The commercial preparation of magnetoplumbite ferrites involves thesolid state reaction of an iron oxide with a divalent metal oxideyielding compound such as the carbonate, hydroxide, or nitrate of lead,strontium or barium at a temperature of 1,000l,300 C. in an airatmosphere. A reactant stoichiometry of MOzFe O (wherein M is Pb, Sr orBa) in the range of 1:4 to 1:55 is commonly used in distinction to thecrystallographic stoichiometry of 1:6. This is done to compensate foriron contamination during subsequent processing as well as possiblevolatization of the divalent metal oxide during the reaction. Theproduct is in the form of crystallites or aggregates of crystallitesranging in size from tens of microns to millimeters. Since the singlemagnetic domain crystallite size is about one micron, this material ismilled to reduce the aggregate and crystal size to this value. As aresult of milling, the powder undergoes a net enhancement of itsintrinsic coercive field as well as having introduced numerouscrystallographic imperfections into the structure. The removal of theseimperfections is required to achieve the desired high magnetic qualityin the sintered magnet.

Heretofore it was common practice to add a few weight percent of animpurity such as silica, alumina, strontium sulfate, bismuth oxide andlead oxide to the unreacted mixture or to the MFe O prior to milling toimprove the magnetic and ceramic quality of the resultant sinteredmaterial. Further, it has been disclosed by Ireland, U.S. Pat. No.2,980,617 that the maximum energy product of the ferrite is increased byadding, prior to compacting, various refractory oxides such as silica,alumina, titania, boric oxide and mixtures such as alumina-silica,calcium oxide-silica, calcium oxidetitania, alumina-titania, chromicoxide-silica, and chromic oxide-titania. In the case of the physicalmixtures, the compounds are preferably combined in equal parts byweight. The preferred materials disclosed in the alumina-silicacombination are represented by an aluminum silicate such as asilica-alumina porcelain, kaolin, and mullite or other clays or mineralsconsisting essentially of aluminum silicates. Unfortunately, in themethod heretofore proposed, some of the MO from the ferrite mixture isleached out into the liquid phase increasing the ferric oxide contentresulting in a dilution of the magnetic properties, i.e. the residualinduction.

In accordance with the present invention, we have discovered a method ofmaking a magnetoplumbite ferrite having the formula MFe O wherein M isselected from the group consisting of strontium and barium, comprisingthe steps of forming an aqueous slurry of the magnetoplumbite ferriteand a liquid phase sintering additive. orienting the ferrite to alignthe crystals, wet pressing the mixture to form a green compact,sintering the green compact at a temperature of about 1,1001,300 C. fora time of about 5 minutes to 8 hours and then magnetizing the sinteredferrite.

The magnetoplumbite ferrites, i.e., BaFe, O, and SrFe O may be preparedby the reaction of ferric oxide and the corresponding alkaline earthmetal carbonate in a molten salt solvent. The molten salt solvent isthen leached from the product to give a monodispersed powder withmaximum crystallite dimensions of about one micron. Another procedurefor preparing the magnetoplumbite ferrites in the form of micron andsubmicron size particles involves reacting the cation oxides of theferrites in a solvent of sodium chloride and potassium chloride in thepresence of a source of water and is disclosed in U.S. Pat. applicationNo. 310,354 filed on Nov. 29, 1972 now U.S. Pat. No. 3,793,443 andassigned to the assignee of the present invention. A further method ofmaking the magnetoplumbite ferrites is disclosed in U.S. Pat.application No. 323,084, filed on Jan. 12, 1973 now U.S. Pat. NO.3,810,973 and assigned to the assignee of the present invention whereinan aqueous solution of ferric chloride is reacted with an alkaline metalhydroxide to form ferric oxide hydrate in a salt solution and then analkaline earth carbonate is added to the mixture. After drying, theferric oxide in the alkaline earth metal carbonate are reacted in amolten salt solvent and the particulate ferrite is recovered.

The liquid phase additives useful in our invention must all contain adivalent metal oxide selected from the group consisting of lead oxide,barium oxide and strontium oxide. The presence of the divalent metaloxides is necessary to avoid the leaching of these oxides from themagnetoplumbite ferrite. The liquid phase sintering additives useful inour invention are represented by the following systems:

1. PbO-Al O -SiO The composition PbOzSiO varies from 1:4 to 4:1. The A10 may be added as PbAl O and constitutes 0.0 to 50.0 weight percent ofthe total additive composition.

2. PbO-Al O -B O The composition PbO-B O varies from 1:4 to 4:1. The A10 may be added as PbA1 O and constitutes 0.0 to 50.0 weight percent ofthe total additive composition.

3. BaO-Al O -B O The additive composition varies from BaSiO to BaO6SiOThe A1 0 may be added as BaAl O and constitutes 0.0 to 50.0 weightpercent of the total additive.

The additives in this system have the composition xBaB O -(1-x)BaSiOwherein x is equal to 0.0 to 1.0; with A1 0 being added as BaAl O in anamount up to 50 weight percent of the total additive composition.

5 SrO-Al O -SiO The additive compositions varies from SrSiO to SrO-6SiOThe A1 0 may be added as SrAl O and constitutes 0.0 to 50.0 weightpercent of the total additive.

The additives in this system have the composition ySrB O -(1-y)SrSiOwherein y is equal to 0.0 to 1.0; with A1 0 being added as SrAl O in anamount up to 50 weight percent of the total additive.

The amount of the additives required to give the beneficial effectduring liquid phase sintering is generally in the range of about 0.5-6.0percent by weight.

The liquid phase additives may be prepared by dry mixing the componentsand heating them in covered platinum crucibles to their melting points.The products are then ground to -325 mesh and remelted to form ahomogeneous composition. The vitreous material obtained is then regroundto 325 mesh and combined with the magnetoplumbite ferrite. in thepreferred method of making the sintered products by our invention, themagnetoplumbite ferrites are milled with the liquid phase additive in asteel ball mill. A milling fluid, such as methanol, is used which may besubsequently removed by volatization. The milling step reduces theparticle size of the additive and in addition homogenizes themagnetoplumbite ferrite and the additive mixture to a higher degree thanobtained by simple dry mixing. The composition is then wet pressed andoriented to align the crystals and to form a green compact. Typically,pressing is performed at a pressure of about 200020,000 psi using asteel die. The samples are thereafter placed in a furnace and fired at atemperature of about l,ll,300 C. in an air or pure oxygen atmosphere fora time of about minutes to 8 hours.

The products obtained by the process of the present invention shouldhave a density of 90 percent of theoretical or greater and should haveproperties which are equivalent or superior to the industry standardsfor BaFe, O sintered magnets (Ceramic 5) and SrFe O sintered magnets(Ceramic 7) as shown in the following table:

TABLE I Ceramic 5 Ceramic 7 EXAMPLE 1 Using a steel ball mill, 30 g. of(molten salt synthesized) BaFe O and 1.2 g. of BaB O BaSiO were milledin the presence of methanol as a milling fluid for 24 hours. Thehomogeneous mixture was then removed from the mill and the methanolevaporated off. The resultant powder having a particle size of s 1.0;1.was then slurried in water, and wet pressed in conventional equipment atpressures of 2000-20000 psi to form green samples.

The samples were thereafter tired in an oxygen containing atmosphere ata temperature of l,l00-l,300 C. for 5 minutes to 8 hours.

The magnetic properties of the magnets obtained after sintering weretypically as follows:

II II ll ll A control experiment was performed by preparing severalsamples of pure BaFe O free of any additives, using several differentpressing pressures and sintering temperatures. There was nodensification at temperatures s l300 C. The samples were chalky andcould be crushed easily into a coarse powder. Chemically pure, 1.0;1.BaFe O therefore, will not densify below l300 C. without sinteringagents.

EXAMPLE ll Following the procedure of Example I, 30.0 g. of BaFe O weremilled with 0.6 g. BaSiO The homoge neous mixture was pressed and thensintered at l,2lO C. for 2 hours in an air atmosphere. The magneticproperties of the product were as follows:

Br 4000 Gauss H 2350 Oersted .1 2400 Oersted )mu r 2.8Xl0 Gauss OerstedEXAMPLE Ill Following the procedure of Example I, 30.0 g. of BaFe O weremilled with 1.2 g: of an additive consist ing in weight percent of BaB O'BaSiO and 10% BaAl O in methanol. The methanol was dried off and thesample was slurred in water, oriented and wet pressed. The sample wassintered at 1,1 15 C. for 2 hours. The magnetic properties were asfollows:

H. 2975 Oe.

( H),... 3.2X10"G. Oe.

EXAMPLE IV Following the procedure of Example I, the additivecompositions, PbO:SiO or PbO:B O were varied from 1:4 to 4:1. The A1 0was added as PbAl O and constituted 0.0 to 50.0 w/o of the totaladditive composition. Two and 4 w/o of the additive in BaFe O sampleswere prepared, and air fired for 2 hours at sintering temperature ofl,l00, l,l50, l,2 OO, l,250 and l,300 C.

The microstructures varied from small, 1 .011. grains in the 1,l00 C.,relatively low-density samples to large, multidomain crystallites in the1,300" C., relatively high-density samples. Assuming a limitation tosingle domain size of s 1.0a, a density greater than 88% could not beattained without simultaneous grain growth to multidomain size. Inaddition, variations of the PbOzSiO or PbO:B O ratio resulted in onlyminor variation in both density and microstructure. The PbAl- Ocontaining additives better maintained the s 1.0;1. grain size with onlya small decrease in density when compared to an Al O -free sampleprepared under the same sintering schedule. Hence, A1 0 is an importantfactor in microstructure control in these PbO-based liquid-phasesintering additive systems.

Anisotropic magnets were prepared from powder samples which gave thebest isotropic magnet densities and microstructures. Table II gives themagnetic properties for those samples prepared with high PbAl O contentadditives.

TABLE II TABLE IV MAGNET PROPERTIES OBTAINED USING ADDlTlVES IN THESYSTEMS PbOAl O -,SiO

A 62.5 w/o PhSiO; 37.5 w/u PbAl O 4 w/u additive, fired alt ll85 (Y for2 hours in air.

B 50 w/n PhSiO 50 wlo PhAI O 4 win additive. fired at l 185' C. for 2hours in air.

EXAMPLE V Following the procedure of Example I, the additivecompositions were varied from BaSiO to BaO6SiO The M 0 was added as BaAlO and constituted 0.0 to 50.0 w/o of the total additive composition. Thequantity of additive in the ferrite was varied from 1.0 to 6.0 w/o.Representative data are given in Table III to show the effects ofvariations in the BaOzSiO ratio as well as additions of BaAl O Allsamples were sintered for 2 hours in an air atmosphere at the indicatedtemperature. These data show that the degree of densification increasesrapidly with temperature. There is some difference in density as afunction of the BaO:SiO ratio, but primarily at lower sinteringtemperatures. The addition of BaAl O reduces the degree of densificationat lower temperatures, but the effect nearly disappears at the highertemperatures. The present system results in slightly lower densityvalues than did the PbO-based additives.

TABLE Ill MAGNETIC DATA FOR ANlSOTROPlC BaFe O MAGNETS PREPARED WITHADDlTIVES IN THE SYSTEM BaOAl O SiO,

C D E Br (6.) 3950 3900 3650 H,- (Oe.) 2425 2425 3200 H (0e 2450 24753275 (BHL 3.6 3.6 3.2 ()(10 G. De.)

C 4.0 w/o BuSiO sintered at I240 C, hours. in air. D 2.0 w/o BuO'3SiOsintered at l l40 C. 2 hours. in air. E 4 wlo (62 w/o BuSiO 37.5 wlo821M 0 sintered at l I85 C.. 2 hours, in air.

EXAMPLE Vl.

Following the procedure of Example I, the BaFe, O, was milled with 4 w/oof an additive consisting of 75 w/o SrB O -SrSiO and 25 w/o SrAl O Thehomogeneous mixture was pressed and then sintered at l,150 C. for 2hours. The magnetic properties of the product were as follows:

Br =3550 G.

(BH),,,,,, =3.l X 10 G. Oe.

The resulting samples were similar to those using the BaO-basedadditives. It is likely that during the course of densification, some ofthe BaO replaced SrO in the additive, the SrO being deposited as SrFe OBaFe O solid solution from the liquid phase.

SINTERED DENSlTlES OF lSOTROPlC BaFe O MAGNETS USING ADDlTlVES lN THESYSTEM BaOAl O -;SiO

Wk) BaSiO Examination of the sample microstructures showed excellentcontrol of grain size in the temperature range 1,100 to l,250 C., andonly at 1,300 C. was there evidence of large, multidomain crystalliteformation. There is little difference in the microstructure attained byvarying the BaO:SiO ratio, but the addition of BaAl Q, aids greatly inthe preservation of a small crystallite size.

Anisotropic sintered magnets were prepared from selected powder samples.Table IV gives the data for those samples. The addition of BaAl Oresults in a lower density; hence lower Br and (Bl-U but yields a markedincrease in H and H These results indicate that BaAl O controls themicrostructure and/or crystallite morphology better than the Al O -freeadditives and therefore results in fewer defects that give rise to lowvalues for H and H EXAMPLE Vll Following the procedure of Example 1,samples were prepared using commercial grade SrFe O which from lotanalysis contained 0.5-1.0 weight percent SiO To the ferrite was added 2weight percent of an additive represented by the formula BaB O 'BaSiOThe magnets were then prepared as described in Example l above and themagnetic properties compared to those of a sample containing noadditive. The comparative data is as follows:

Br (6.) 3350 3450 H (Oe.) 3250 3300 H (Oe.) 4350 4575 H) 2.8 3.0

F SiO,. 2 wlo AI Q, in its-received material. No additive present.sintered at I200 C. for 2 hours in air.

6 Same as F. but with 2 W10 BaB,O -BuSiO; added.

From our experiments, we have been able to draw the followingconclusions:

1. The presence of a liquid phase additive is required to optimize thedensification and microstructural development of MFe O magnets.

2. Liquid phase compositions in the systmes MO- Al O -SiO and MO-Al O -BO -SiO- wherein M is a member selected from the group Pb, Sr, and Baproduce dense, sintered magnets with excellent magnetic properties.

3. A1 is important in the control of the compact microstructure, andpossibly crystallite morphology, which results in high values of H and Hwith only a minor reduction in Br.

4. The quantity of liquid phase agent is important in that insufficientquantities result in less than perfect microstructure and morphologycontrol, even though a high degree of densification is possible.

It will be appreciated that the invention is not limited to the specificdetails shown in the examples and illustrations and that variousmodifications may be made within the ordinary skill in the art withoutdeparting from the spirit and scope of the invention.

We claim:

1. In a method of making a permanent magnet from a magnetoplumbiteferrite powder having the formula MFe O wherein M is selected from thegroup consisting of strontium and barium, including the steps oforienting the ferrite powder to align the crystals and wet pressing thepowder to form a green compact, sintering the green compact at atemperature of about llOO-1300 C. for a sufficient time to form a bodyhaving a density of at least 90% of the theoretical density andmagnetizing the sintered ferrite composition, the improvement comprisinghomogeneously dispersing in the ferrite powder 0.5-6.0 percent by weightof a liquid phase sintering additive selected from the group consistingof the following systems:

2. The method of claim 1, wherein said ferrite is BaFe O 3. The methodof claim 1, wherein said ferrite is 511 6 20

1. IN A METHOD OF MAKING A PERMANENT MAGNET FROM A MAGNETOPLUMBITEFERRITE POWDER HAVING THE FORMULA MFE12 019, WHEREIN M IS SELECTED FROMTHE GROUP CONSISTING OF STRONITUM AND BARIUM, IMCLUDING THE STEPS OFORIENTING THE FERITE POWDER TO ALIGN THE CRYSTALS AND WET PRESSING THEPOWDER TO FORM A GREEN COMPACT, SINTERING THE GREEN COMPACT AT ATEMPERATURE OF ABOUT 1100*-1300*C. FOR A SUFFICIENT TIME TO FORM A BODYHAVING A DENSITY OF AT LEAST 90% OF THE THEORETICAL DENSITY ANDMAGNETIZING THE SINTERED FERRITE COMPOSITION, THE IMPROVEMENT COMPRISINGHOMOGENEOUSLY DISPERSING IN THE FERRITE POWDER 0.5-6.0 PERCENT BY WEIGHTOF A LIQUID PHASE SINTERING ADDITIVE SELECTED FROM THE GROUP CONSISTINGOF THE FOLLOWING SYSTEMS: BAO-A12O3-SIO2 BAO-A12/3-B2O3-SIO2SRO-A12O3-SIO2 SRO-A12O3-B2O3-SIO2
 2. The method of claim 1, whereinsaid ferrite is BaFe12O19.
 3. The method of claim 1, wherein saidferrite is SrFe12O19.